I may be wrong but I don't think a TD can be arrested while on his way to the Dail.

I thinks that's correct. He obviously can't smoke in the Dail itself because of the smoking ban, and he can't be arrested on his way to or from Dail. So once he is'nt actually driving himself to or from the Dail (Driving a Motor Vehicle while under the Influence), he should be able to splif away to his heart's content while somebody else drives him (not on public transport though) or while he walks to and from the Dail.